There are more and more types of electronic devices. Some electronic devices have their own batteries to supply power required for their operations, other electronic devices can only operate with external power sources, and there are also some electronic devices that can be powered by either internal batteries or external power sources.
Electronic devices can be inter-connected via a physical interface to communicate with each other based on a certain protocol. According to some protocol (e.g., Universal Serial Bus, or USB, protocol), once two electronic devices are connected, it is necessary to determine one of them as a host device and the other as a slave device according to procedures defined in the protocol. The electronic device acting as the host device needs to supply power to the slave device, so as to activate the slave device and then control it (e.g., reading data from or writing data into the slave device).
However, such power supply scheme is not sufficiently flexible in some situations. For example, a mobile phone of a user typically has many files stored therein and the user can process these files on an operation interface of the mobile phone. If the user wants to output these files via an external device (e.g., to play a music file currently being played on the mobile phone via a vehicle audio system) while processing these files, he/she needs to connect his/her mobile phone to the vehicle audio system via a physical interface (such as a USB interface) such that the audio signal from the mobile phone can be output to the vehicle audio system. In this case, the mobile phone acts as a host device and the vehicle audio system acts as a slave device. According to the existing protocol procedure, after the vehicle audio system is connected to the mobile phone, the mobile phone acting as the host device will supply power to the vehicle audio system acting as the slave device.
Because the mobile phone has limited power, the vehicle audio system is self-powered and its power consumption is much higher than that of the mobile phone, it is thus unreasonable for the mobile phone to supply power to the vehicle audio system as in that case the battery lifetime of the mobile phone would be significantly reduced.
Therefore, there is a need for a method capable of providing reasonable power supply control between electronic devices.